Calculate Watt Hours Fast: Turn a 20,000mAh Power Bank into 74 Wh

Watt-hours equal amp-hours multiplied by voltage: Wh = Ah × V. For batteries labeled in milliamp-hours, divide by 1,000 first — so a 20,000mAh power bank with 3.7V cells holds 20 Ah × 3.7V = 74 Wh. For a device rated in watts, multiply by run time (Wh = W × h), and divide a battery's Wh by the device's watts to get hours of runtime. The rest of this guide covers the details that trip people up: which voltage to use, why you never get the full labeled capacity, and what that means for choosing a battery size.

The Formulas You Need

You have	Formula	Example
Amp-hours (Ah) and volts	Wh = Ah × V	100Ah × 12V = 1,200Wh
Milliamp-hours (mAh) and volts	Wh = mAh × V ÷ 1,000	20,000 × 3.7 ÷ 1,000 = 74Wh
Watts and hours of use	Wh = W × h	40W × 8h = 320Wh
Battery Wh and a device's watts	Hours = Wh ÷ W	1,200Wh ÷ 100W = 12h (theoretical)
Wh to kWh (for utility-bill comparisons)	kWh = Wh ÷ 1,000	1,200Wh = 1.2kWh

Voltage matters because watt-hours measure energy: two batteries with the same amp-hour rating store very different amounts of energy if their voltages differ. You'll also see "watts per hour" used online — that's almost always a mix-up, since power is already a rate (watts) and energy is watt-hours. If the watts-versus-watt-hours distinction is still fuzzy, start with our watt-hours vs. watts beginner's guide and come back.

The Voltage Mistake Almost Everyone Makes

A power bank's USB port outputs 5V, but the cells inside run at about 3.7V (nominal) — and the mAh number on the label is measured at the cell voltage. So to find the energy stored, multiply by 3.7V, not 5V. Using 5V would inflate a 20,000mAh bank to 100Wh instead of 74Wh, a 35% error that matters a lot when you're checking airline limits or comparing against a power station. Some banks use 3.6V or 3.85V cells, so if the label prints a Wh figure, trust that over your own calculation.

Common Power Banks and Power Stations, in the Same Units

Putting everything in watt-hours makes comparisons straightforward:

Battery	Wh	Equivalent mAh at 3.7V
10,000mAh power bank	37Wh	10,000mAh
20,000mAh power bank	74Wh	20,000mAh
Typical smartphone battery (4,500mAh at ~3.85V)	~17Wh	~4,700mAh
EcoFlow River 3	245Wh	~66,000mAh
Bluetti AC70	768Wh	~208,000mAh
Jackery Explorer 1000 v2	1,070Wh	~289,000mAh

That last row is why a power station and a power bank aren't really the same product category — see our power bank vs. power station breakdown for what the extra capacity (and the built-in AC inverter) actually buys you.

Why a 20,000mAh Bank Doesn't Charge Your Phone 4 Times

On paper, 74Wh against a ~17Wh phone battery looks like four-plus full charges. In practice you'll get about three, because the bank has to step its 3.7V cell voltage up to 5V (or higher for fast charging) and every conversion wastes some energy as heat. Voltage conversion typically runs around 80–90% efficient, and after cable and charging losses the usable energy commonly lands at roughly 60–85% of the rated figure — Anker's support documentation covers why a power bank's delivered capacity falls short of its rated capacity for its own products. For your 20,000mAh bank, that works out to roughly 9,000–12,500mAh of usable capacity at 5V.

Power Bank Wh and Airline Rules

Airlines regulate lithium batteries in watt-hours, not mAh, which is exactly why this conversion is worth knowing before you pack. The FAA's PackSafe guidance allows power banks up to 100Wh in carry-on without special approval, and 101–160Wh only with airline approval:

Rated capacity	Wh at 3.7V	Carry-on status
10,000mAh	37Wh	Allowed
20,000mAh	74Wh	Allowed
26,800mAh	~99Wh	Allowed (right at the limit)
30,000mAh	111Wh	Needs airline approval

Roughly 27,000mAh at 3.7V is the practical ceiling for the 100Wh rule. Spare lithium batteries must travel in carry-on, never checked baggage. Our flying with a power station guide covers the full rules, including why nearly every real power station is too big to fly.

Why Usable Watt-Hours Are Lower Than the Label

The capacity printed on any battery is a best-case number. Planning guidance from the University of Colorado Denver's battery backup program is blunt about it: you will never get the rated capacity of a battery system — expect 80–90% of the rated value. The usual culprits:

• Inverter and conversion losses when turning DC battery power into AC output. Our runtime calculator applies an 85% efficiency factor for this reason.
• Depth-of-discharge limits that protect the battery by leaving some capacity untouched.
• Cold temperatures, which temporarily shrink usable capacity — roughly 85–90% of room-temperature capacity at freezing. See our cold weather performance guide.
• Heat and aging, which cause permanent capacity loss over time — covered in our extreme heat guide and battery lifespan guide.

From Watt-Hours to the Right Battery Size

Once everything is converted to Wh, picking a capacity takes two adjustments on top of your daily total: divide by your usable fraction (85% for a power station with an AC inverter), then add a 20–30% margin for cold weather, aging, and devices you forgot. For example, a daily need of 810Wh — a CPAP (320Wh), Wi-Fi router and modem (240Wh), LED lighting (150Wh), and phone and laptop charging (100Wh) — becomes 810 ÷ 0.85 ≈ 953Wh, or roughly 1,150–1,250Wh with the margin. A unit like the Jackery Explorer 1000 v2 (1,070Wh) covers that base load with limited margin; a 2,000Wh-class unit gives comfortable headroom.

Capacity isn't the whole story, though: a battery with enough Wh can still fail to start a device with a high startup draw, so also check continuous and surge output — see our running watts vs. surge watts guide. For the full step-by-step process of inventorying your devices and sizing by use case, use our wattage sizing guide. The solar recharge time guide turns your daily Wh into the solar input you'd need to refill it, and if a device label lists amps instead of watts, our amps to watts guide handles that conversion first.

Measure Real Draw Instead of Trusting the Label

Printed wattage is usually a maximum or a rounded figure, and devices with compressors or heating cycles draw very differently over time. A plug-in watt meter (or the output display on most power stations) shows your actual draw in seconds. Measure first, then build the 20–30% margin on top of the real number rather than a guess — it's the cheapest insurance against running short at the worst moment.

Frequently Asked Questions

How many watt-hours is a 20,000mAh power bank?

About 74Wh, calculated as 20 Ah × 3.7V for typical lithium-ion cells. The exact figure depends on the manufacturer's actual cell voltage, so check for a Wh rating printed on the label. Usable energy after voltage-conversion losses is lower, commonly 60–85% of that.

Why does my 20,000mAh power bank only charge my phone about three times?

Because the labeled mAh is measured at the 3.7V cell voltage, and delivering that energy at 5V or higher through a boost converter and cable wastes 15–40% as heat. A 74Wh bank delivering roughly 45–63Wh to a 17Wh phone battery gives about three full charges, not four or five.

How long is 1,500 watt-hours?

It depends on the load. At 85% usable capacity, 1,500Wh gives about 1,275Wh of real energy: roughly 12.75 hours for a 100W load, about 32 hours for a 40W CPAP (around four nights), or about 8.5 hours for a 150W refrigerator.

How long is 1,200 watt-hours?

A 1,200Wh battery — equivalent to a 12V, 100Ah battery — would theoretically power a 100W load for 12 hours (1,200 ÷ 100). Real runtime is shorter once you account for inverter losses and depth of discharge, so plan on roughly 10 hours at 85% usable.